FreeBSD_11 specific fix

FreeBSD_11 specific fix

C11 mandates the definition of timespec_get() and TIME_UTC.
However, FreeBSD11 announce C11 compliance, but does not provifr timespec_get(),
breaking link stage for benchfn.
Since it does not provide TIME_UTC either, which is also required by C11,
test this macro: this will automatically rule out FreeBSD 11 for this code path
(it will use the backup C90 path instead, based on clock_t).

The issue seeems fixed in FreeBSD 12.